Connector Insertion Force, Normal Force and Contact Wear

A connector is a mechanical device that carries an electrical signal, and it fails for mechanical reasons far more often than for electrical ones. The force that holds the two halves together, the force needed to push them together, and the wear that accumulates on the contact surfaces together decide how long the interface will last. This article explains the relationship between those forces, the plating systems that resist wear, the corrosion mechanisms that do not, and the rules that keep a connector reliable through its service life.

What Makes a Connector Work

A contact interface works because two metal surfaces are pressed together hard enough to break through the surface films and create a metallic connection at a number of small asperities. The true contact area is a tiny fraction of the apparent area, and it is proportional to the applied force rather than to the size of the surfaces.

That is why force is the primary design variable. A contact with plenty of force has a low and stable resistance even if the surfaces are not perfectly clean, while a contact with marginal force depends on the surface condition and drifts as the films grow.

Normal Force and Contact Resistance

The normal force is the load pressing the two contact surfaces together, and it is generated by the spring element inside the connector: a cantilever beam, a tuned spring, a hyperboloid cage or a dual beam contact. Contact resistance falls as the normal force rises, rapidly at first and then more slowly once the asperities have been deformed.

The design target is a force high enough to be stable over the life of the connector and low enough that the insertion force stays within the specification. Those two requirements pull in opposite directions, and the balance is what the connector designer optimises.

Insertion and Extraction Force

The insertion force is the sum of the forces on every contact plus the friction of the housing, the guide features and any polarising keys. It is measured in newtons per contact in the specification and felt by the user as the total, which means a high contact count connector can have a comfortable per contact figure and an unacceptable total.

The ratio of extraction force to insertion force is a useful indicator of the contact design. A very high extraction force suggests a robust contact but makes the connector hard to unmate, while a very low one suggests that the normal force may be too small. Both figures should be specified with the mating condition and the number of cycles already applied.

Plating Systems and Wear

Contact surfaces are plated because the base metal, usually a copper alloy, oxidises readily. Gold is the standard for high reliability because it does not form a significant oxide and its resistance is stable at low force. The gold layer is thin and is backed by a nickel barrier that stops the base metal from diffusing through it.

The thickness of the gold determines how many mating cycles the connector can survive before the underlying layers are exposed. A thin flash of gold is fine for a connector that is mated once, while a connector that is cycled repeatedly needs a thicker layer. Our plating thickness guide covers how the layers are specified and measured.

Wear Mechanisms Over Mating Cycles

Each mating cycle slides the two surfaces against each other over a small distance, and that sliding wears the plating. The wear pattern removes gold from the contact area, exposes the nickel barrier and eventually exposes the base metal. Once the base metal is exposed, the contact resistance rises and becomes sensitive to the environment. This progressive contact wear is the limiting factor for any connector that is cycled repeatedly in service.

The wear rate depends on the normal force, the sliding distance and the hardness of the surfaces. A high normal force increases wear but produces a more stable contact, while a low force produces less wear but a contact that is vulnerable to films. The design has to pick a point that is acceptable for both.

Fretting Corrosion

Fretting is the small amplitude relative motion that occurs between two contact surfaces under vibration or thermal cycling. It damages the plated surface and produces oxide debris that accumulates in the contact area, and the result is a resistance that rises over time and eventually becomes an intermittent open circuit.

Fretting is worst where the normal force is low and the motion is small, because the debris stays in place instead of being swept away. A higher normal force, a lubricant, or a contact geometry that concentrates the motion in one area are the usual mitigations. A connector exposed to vibration and thermal cycling at the same time is the case most likely to fail. The damage it produces is a form of contact wear driven by motion rather than by the number of mating cycles.

Effect of Board Thickness and Alignment

The board thickness, the hole size and the position of the connector housing all affect the force the connector applies. A pin inserted into a plated through hole with a press fit depends on the hole tolerance and the plating thickness, and a variation in either changes the retention and the stress in the barrel.

Alignment matters as much. A connector that is not square to the board or that has to be forced into position applies a side load to the contacts, which changes the normal force and accelerates wear. Our component tolerance notes describe how mechanical tolerance stacks affect reliability.

Test Methods and Acceptance Criteria

Contact resistance is measured with a four wire method at a specified current, and the specification should state the test current, the dry circuit condition and the maximum resistance. A dry circuit measurement at low voltage is the appropriate one for a signal contact, because it does not break through films that a power contact would.

Insertion and extraction force are measured with a tensile tester at a defined speed, and the result is reported for the first cycle and after a defined number of cycles. Reporting the force after several cycles rather than only on a new connector shows whether contact wear is eroding the retention, which is far more informative than a single first cycle figure. Our quality guide describes how a contact defect found at incoming inspection is classified.

Design and Assembly Rules

The rules that keep a connector reliable are straightforward. Select the plating thickness from the number of mating cycles expected, keep the normal force high enough that the contact is stable at the end of life, support the connector housing so that mating does not load the solder joints, and specify the insertion force per contact as well as the total.

At gopcb the surface condition the contact will sit on is controlled through our surface finish documentation, and the current the contact has to carry is checked against our current capacity guide so that the connector rating and the board are consistent.

FAQ

How thick should the gold plating be? It depends on the number of mating cycles: a flash for a single mating, a thin layer for occasional cycling and a thicker layer for a connector that is mated repeatedly. The supplier specification should be matched to the application rather than assumed.

Is a higher insertion force better? Up to a point, because it implies a higher normal force and a more stable contact. Beyond that point the connector becomes difficult to mate and the housing and solder joints see unnecessary stress.

What causes an intermittent open in a connector that tested good? Fretting corrosion is the most common cause, and it appears only after the product has seen vibration and thermal cycling. Reviewing the normal force and the plating system is the place to start.